#00060900

Strong positivity for CD99 confirms involvement by Ewing sarcoma in this staging bone marrow biopsy from an adolescent male.  CD99 is the arguably the single best immunostain for Ewing sarcoma but should never be performed in isolation, as it is not specific for Ewing sarcoma and instead expressed normally by T cells and activated B cells, and in disease by both T and B lymphoblastic leukemia/lymphoma (a potential pitfall given a similar morphologic appearance), anaplastic large cell lymphoma and other lymphomas, acute myeloid leukemia and extramedullary myeloid tumor, and a large number of other cell types and tumors